High ion density dry etching of compound semiconductors

The use of plasma sources that generate high ion densities (> 10¹¹ cm⁻³) enables dry etching of compound semiconductors at high rates with anisotropic sidewalls. In this paper we review the use of several types of electron cyclotron resonance (ECR) plasma sources and contrast the result with those obtained under reactive ion etching conditions. Various problems occurring in dry etching will be discussed, including aspect ratio dependent etch rates, mask erosion, sidewall roughening and damage introduction into the semiconductor. This damage may consist of point and line defect creation, non-stoichiometric surfaces, resputtering of mask materials or deposition of contaminating films. The use of low or high substrate temperatures to control the desorption kinetics of etch products is also discussed; at low temperatures problems can occur with condensation of the etch gases onto the substrate, while at elevated temperatures it is necessary to thermally bond the sample to the r.f. powered electrode to obtain reproducibility. Etch selectivity between the components of heterostructure systems such as GaAs/AlGaAs, GaAs/InGaP, InGaAs/AlInAs and GaN/AlN is usually much worse under high ion density conditions because of the high rates and large physical component.     

Wavelength dependent laser-induced etching of Cr-O doped GaAs: Morphology studies by SEM and AFM

The laser induced etching of semi-insulating GaAs 〈100〉 is carried out to create porous structure under super- and sub-bandgap photon illumination (h v). The etching mechanism is different for these separate illuminations where defect states play the key role in making distinction between these two processes. Separate models are proposed for both the cases to explain the etching efficiency. It is observed that under sub-bandgap photon illumination the etching process starts vigorously through the mediation of intermediate defect states. The defect states initiate the pits formation and subsequently pore propagation occurs due to asymmetric electric field in the pore. Formation of GaAs nanostructures is observed using scanning electron (SEM) and atomic force microscopy (AFM).     

Micro structuration of gaas surface by wet etching: towards a specific surface behavior

Resonant microelectromechanical systems are promising devices for real time and highly sensitive measurements. The sensitivity of such sensors to additional mass loadings which can be increased thanks to the miniaturisation of devices is of prime importance for biological applications. The miniaturisation of structures passes through a photolithographic process and wet chemical etching. So, this paper presents new results on the anisotropic chemical etching of the gallium arsenide (GaAs) crystal used for this application, in several solutions. This paper focuses on the micro/nanostructuration of the sensing surface to increase the sensor sensitivity. Indeed, this active surface will be biofunctionalized to operate in biological liquid media in view of biomolecules detection. Several experimental conditions of etching bath composition, concentration and temperature were examined to obtain a large variety of geometrical surfaces topographies and roughness. According to the orientation dependence of the chemical etching process, the experiments were also performed on various GaAs crystal plates. The bath 1 H3PO4:9 H2O2:1 H2O appeared to be particularly adapted to the fabrication of the GaAs microstructured membrane: indeed, the bath is highly stable, anisotropic, and, as a function of temperature, it allows the production of a large variety of GaAs surface topographies.     

Three-dimensional fabrication on GaAs surfaces using electron-beam-induced carbon deposition followed by wet chemical etching

We propose a method for fabricating three-dimensional structures on GaAs surfaces using electron beam (EB) irradiation followed by wet chemical etching. An etch-resistant hydrocarbon layer forms on the GaAs surface with the EB irradiation. Structures can be fabricated after etching using the hydrocarbon layer to block the etching. The height dependence on the irradiation and etching conditions was investigated as a means of controlling the height of the structures. A higher structure was fabricated at higher doses. The etching selectivity changed with the concentration of the etchant. A three-dimensional structure was fabricated based on these results, demonstrating the possible use of this method as a novel three-dimensional fabrication method for GaAs surfaces.     

Application of Nomarski differential interference contrast microscopy to highlight the prior austenite grain boundaries revealed by thermal etching

Revealing prior austenite grain boundaries by thermal etching has been demonstrated to be a reliable and fast method compared to chemical etching for microalloyed carbon steels. However, sometimes visualization of the thermally etched prior austenite grain boundaries is hindered by the presence of grain boundaries of other phases (e.g. ferrite and/or pearlite) which are thermally etched during slow cooling from high temperature. This work shows that, under these conditions, the use of Nomarski differential interference contrast microscopy under bright field illumination helps to highlight the thermally etched prior austenite grain boundaries.     

How is wettability of titanium surfaces influenced by their preparation and storage conditions?

The effect of two different etching procedures with inorganic acids (HSE and CSE)—one using additionally strongly oxidising conditions due to the presence of CrO₃ (CSE)—and consecutive storage conditions (dry methanol and air) for previous corundum blasted titanium surfaces is compared with respect to their wettability behaviour and the potential of the etching processes for removing remaining blasting material. The etching procedures result in distinct different surface morphologies. Whereas the HSE surface shows sub-mm to sub-μm structures but neither porosity nor undercuts, the CSE surface is extremely rugged and porous with structures protruding the more homogeneously attacked areas by several micrometers. By EDX analysis both remaining blasting material and chromium and sulphur from the etching treatment has been detected on the CSE surfaces only. Both surfaces states show super-hydrophilic behaviour immediately after etching and storage up to 28 days in dry methanol. Whereas contact with air does not change super-hydrophilicity for the CSE samples, wettings angles of the HSE samples increase within minutes and reach about angles of about 60° and 90° after one and 2 days exposure to air, respectively. The increasing hydrophobicity is discussed with respect to the formation of a surface coverage from hydrocarbons originating from aromatic compounds present in traces in air.     

Chemical etching of (1 0 0) GaAs in the HNO3-HF-H2O system

This paper describes the action of solutions containing nitric acid, hydrofluoric acid and water on GaAs of (1 0 0) orientation. Single-crystal slices of the semiconductor were immersed in the etch and the rate of dissolution was measured. Results were taken over a wide range of composition of the solution. The results show a marked resemblance to the etch-rate data for silicon in the same etching solution, although the actual etching rates for GaAs were lower. The semiconductor surfaces were observed carefully after etching, using techniques of optical microscopy and scanning electron microscopy. Etch hillocks were observed for many compositions of the etch. It was found that the hillocks were covered by a layer of oxide: if any of this oxide covering came off, the hillock quickly disappeared. Sequential etching experiments showed that the hillocks were not associated with dislocations. Both the etch-rate data and the microscopic examination of the GaAs surfaces suggest that the system is probably diffusion-limited over the range of etch composition studied in this work.     

Optical properties of a strained GaAs/Si heterostructure after rapid thermal annealing

Photoluminescence (PL) and Raman spectroscopy measurements on GaAs/Si heterostructures grown by molecular beam epitaxy were performed in order to characterize the optical properties of the samples after thermal treatment. The crystalline quality of the annealed GaAs/Si was remarkably improved in comparison with that of the as-grown GaAs/Si. The results of PL spectra for rapidly thermally annealed GaAs/Si showed that the peaks at 1.501 and 1.485 eV, which are related to intrinsic bands, and the peak at 1.467 eV, which can be attributed to impurities, were well resolved. The magnitude of the deformation in the GaAs layer, as determined from the PL measurements, was approximately 2.1 × 10^7minus;3. The ratio of the peak intensity of the longitudinal optical phonon to that of the transverse optical phonon for rapidly thermally treated GaAs/Si increased dramatically in comparison with the corresponding rates for the as-grown GaAs/Si. These results indicate that the crystallinity of the GaAs epilayer is improved by thermal treatment.     

Etching of GaAs substrates to create As-rich surface

Several different cleaning procedures for GaAs (100) substrates are compared using X-ray photoelectron spectroscopy and optical microscopy. This work emphasizes the effect of the last etching step: using either HCl, HF-ethanol (5%) or static deionized water after HCl cleaning. All the procedures except HCl solution (1:1) produce an As-rich surface. Also, none of the etchants except HF-ethanol solution produce Ga or As-rich (oxide free) surfaces. Optical microscopic study shows different etch pits produced due to etching in different solutions.     

Six-fold hexagonal symmetric nanostructures with various periodic shapes on GaAs substrates for efficient antireflection and hydrophobic properties

We fabricated various periodic nanostructures with a six-fold hexagonal symmetry on gallium arsenide (GaAs) substrates using simple process steps, together with a theoretical analysis of their antireflective properties. Elliptical photoresist (PR) nanopillars, which are inevitably generated by the asymmetric intensity distribution of the laser interference, were converted to rounded lens-like patterns by a thermal reflow process without any additional complex optic systems, thus leading to an exact six-fold hexagonal symmetry. Various shaped periodic nanostructures including nanorods, cones, truncated cones, and even parabolic patterns were obtained under different etching conditions using the rounded lens-like PR patterns formed by the reflow process. For the parabolic structure, the calculated lowest average reflectance of ～ 2.3% was obtained. To achieve better antireflection characteristics, an aluminum-doped zinc oxide (AZO) film was deposited on the GaAs parabolas, which forms an AZO/GaAs parabolic nanostructure. The structure exhibited a low average reflectance of ～ 1.2% over a wide wavelength region of 350-1800?nm and a hydrophobic surface with a water contact angle of θ(c) ～ 115°. The calculated reflectances were reasonably consistent with the measured results.     

Precise chemical analysis development for Si and GaAs surfaces

Precise chemical analysis (PCA) was developed to allow the study of non-interconnected atoms on crystalline semiconductor surfaces, such as those produced during rapid thermal processing (RTP) of silicon and electron beam lithography on gallium arsenide (GaAs). The PCA method is based on selectively dissolving the different components present on the semiconductor surface using preferential etchant solutions. After etching, the etchant solution, containing the etched component, is analyzed by a photometric technique. In this paper, we present photometric measurements of the amount of free (non-interconnected) atoms that remain on semiconductor surfaces following electron beam and RTP processing. In this context, free atoms are those presenting in any form other than crystalline GaAs or Si, for instance, those in the form of surface oxides. Using the PCA method, free Ga and As were detected on GaAs surfaces after electron beam lithography. Free silicon, boron and phosphorous atoms were found on silicon surfaces after RTP. The concentration of boron diffused into a silicon wafer during RTP was also carried out by means of slight surface etching. We estimate the accuracy of this PCA method at 2% for Ga and 5% for all other elements.     

Evaluation of reactive ion etching processes for fabrication of integrated GaAs/AlGaAs optoelectronic devices

Reactive ion etching (RIE) was used for the fabrication of GaAs/AlGaAs optoelectronic devices (laser diodes and photodetectors) for optical interconnect applications. Smooth, vertical sidewalls with a smooth surface at the field were obtained after optimizing RIE conditions in BCl₃-formed plasma. Accurate in-situ monitoring of the etching process was realized by laser interferometry end-point detection. This led to good process control and reproducibility of the demanding fabrication of the optoelectronic devices. The RIE etching process did not affect the electrical properties of the device by increasing the surface recombination currents. Lasers with etched mirrors exhibited a threshold current density of 970 A cm⁻², which is one of the best values ever reported. The feasibility of a simple technology for the fabrication of optoelectronic circuits, based on a BCl₃ RIE process for laser mirror etching, has been demonstrated.     

Preparation of thin GaAs suspended membranes for gas micro-sensors using plasma etching

The GaAs/AlGaAs heterostructure layer system grown by MBE on GaAs substrate was designed to be used for micro-mechanical structure fabrication. In the first step, double-side aligned photolithography is carried out to define the etching masks on both sides of the substrate. After this, highly selective reactive ion etching (SRIE) of GaAs from the front side is used to determine the lateral dimensions of the membrane structure. The vertical dimension is defined by deep backside SRIE through a 300 μm thick GaAs substrate to the AlGaAs etch stop layer, hence the structure thickness is precisely determined by the thickness of MBE-grown GaAs layer over this etch stop layer. The last step is selective etching of the AlGaAs etch stop layer. The thermal resistance value of the membrane structure as high as 21 K/mW is achieved.     

Facile tuning of superhydrophobic states with Ag nanoplates

GaAs wafers have been decorated with Ag nanoplates through direct galvanic reaction between aqueous AgNO₃ solutions and GaAs, resulting in Ag nanoplate/GaAs composite surfaces with varying hydrophobocity after the Ag nanoplates are coated with self-assembled monolayers of alkyl thiol molecules. By carefully controlling the reaction conditions, such as growth time and concentration of the AgNO₃ solution, the size, thickness, and surface roughness of the individual Ag nanoplates can be tuned in order to produce different topographic structures and roughness of the composite surfaces, which in turn infl uences the hydrophobicity of the surfaces. The as-synthesized composite surfaces have been found to exhibit various levels of hydrophobicity and different wetting states such as the Wenzel wetting state, Cassie impregnating wetting state, and Cassie nonwetting state. The relationship between surface structure and hydrophobic state is also discussed. Electronic Supplementary Material  Supplementary material is available for this article at and is accessible for authorized users. This article is published with open access at Springerlink.com     

Site-controlled growth of InP/GaInP quantum dots on GaAs substrates

A technology platform for the epitaxial growth of site-controlled InP quantum dots (QDs) on GaAs substrates is presented. Nanoholes are patterned in a GaInP layer on a GaAs substrate by electron beam lithography and dry chemical etching, serving as QD nucleation centers. The effects of a thermal treatment on the structured surfaces for deoxidation are investigated in detail. By regrowth on these surfaces, accurate QD positioning is obtained for square array arrangements with lattice periods of 1.25?μm along with a high suppression of interstitial island formation. The optical properties of these red-emitting QDs (λ?～?670?nm) are investigated by means of ensemble- and micro-photoluminescence spectroscopy at cryogenic temperatures.     

Damage-free top-down processes for fabricating two-dimensional arrays of 7 nm GaAs nanodiscs using bio-templates and neutral beam etching

The first damage-free top-down fabrication processes for a two-dimensional array of 7 nm GaAs nanodiscs was developed by using ferritin (a protein which includes a 7 nm diameter iron core) bio-templates and neutral beam etching. The photoluminescence of GaAs etched with a neutral beam clearly revealed that the processes could accomplish defect-free etching for GaAs. In the bio-template process, to remove the ferritin protein shell without thermal damage to the GaAs, we firstly developed an oxygen-radical treatment method with a low temperature of 280?°C. Then, the neutral beam etched the defect-free nanodisc structure of the GaAs using the iron core as an etching mask. As a result, a two-dimensional array of GaAs quantum dots with a diameter of ～ 7 nm, a height of ～ 10 nm, a high taper angle of 88° and a quantum dot density of more than 7 × 10(11) cm(-2) was successfully fabricated without causing any damage to the GaAs.     

Etching of n-GaN - Important step in device processing

Photo-assisted electrochemical (PEC) and photo-assisted electrodeless (ELPEC) etching of n-doped GaN layers grown on sapphire in a KOH based solution under illumination of Hg arc lamp is demonstrated. Smooth surfaces were obtained for a narrow range of etching conditions. It was found out that this window could be extended by using etching conditions which produced “whiskers”. For ELPEC etching final etched surfaces were much smoother with stirring but showed distinct bar objects. Such objects could be eliminated by chopped irradiation when also the etching rate decreased and smoothing of pyramidal and bar objects in the etched surfaces was observed. This effect is most probably caused by the electron-hole pair recombination suppressed at semiconductor dislocation locations.     

Effective diffraction gratings via acidic etching of thermally poled glass

Relief diffraction gratings are formed via acidic chemical etching of a periodically poled soda-lime glass. The thermal poling under 1000 V DC is performed at 325 °C using a thermally stable glassy-carbon anodic electrode with periodic grooves, the depth of the grooves being of ∼650 nm. Poling-induced modification of the glass results in deepening the glass anodic surface in the regions under the ribs of the anodic electrode due to volume relaxation and in increasing chemical durability of these regions in acidic media comparatively to the virgin glass. Chemical etching of the poled glass in NH₄F:8H₂O solution allows additional to the thermal poling shaping of the glass surface via faster dissolution of unpoled/less poled glass regions. The morphology of the glass surface before and after the etching is characterized with atomic force and scanning electron microscopy. About 30 min etching provides the formation of ∼0.9 μm in height relief diffraction gratings with the diffraction efficiency close to the theoretically achievable ∼30% for multi-order diffraction. In vivo measuring of the diffraction efficiency in the course of the etching allows precise fabrication of the gratings.     

Experimental investigation of arsenic enrichment at annealed GaAs-anodic oxide interfaces

The time-temperature annealing conditions which lead to the greatest arsenic enrichment at GaAs-anodic oxide interfaces were investigated, mainly by combining photoluminescence and Rutherford backscattering analysis. It is found that a 20 min treatment at 550°C or a 60 min treatment at 500°C is likely to give the thickest intermixed arsenic-rich layer. These arsenic-enriched GaAs surfaces may be used to prepare GaAs/InAs ohmic contacts after etching the remaining oxide and the post-deposition of an indium metal layer and subsequent alloying treatments.     

Microstructure and electrical characteristics of Ni-Cr thin films

Microstructure of thin films of different compositions of Ni-Cr alloy, over the range from 40 wt.% to 80 wt.% Ni, deposited on silicon nitride coated GaAs substrates, is investigated under Transmission Electron Microscope (TEM) in as-deposited and annealed conditions. The procedure of wet etching vias through GaAs substrates is used for preparing samples for TEM. Microstructures of as-deposited films of different Ni-Cr alloys are related to the nature of the parent alloy in the binary phase diagram. The microstructural transformations in annealed films of single and two-phase alloy materials are interpreted based on the comparison of TEM micrographs in conjunction with the X-ray diffraction and electron diffraction data. The changes in the electrical characteristics such as resistivity and temperature coefficient of resistance (TCR) of the films in relation to the microstructural transformations are discussed. The effect of superimposing a polyimide layer on the TCR of Ni-Cr films is also discussed.